Water Age Distributions Research Articles

Transport timescales for identifying seasonal variation in Bass Strait, south-eastern Australia

A three-dimensional σ-coordinate ocean model with realistic forcing is used to derive and compare flushing time, residence time and water age distributions in Bass Strait, a broad shallow shelf sea in south-eastern Australia which exhibits seasonal variation in water mass properties and circulation. Results illustrate flushing out of most old water occurs each winter–spring period and the circulation in summer increases age of remnant older water in the Strait. A correlation between water age and salinity is found in winter which is the result of advective “erosion” of the interior water mass. The presence of an area in the south-eastern part of the interior with low flushing is suggested to result from a number of physical factors including the presence of quasi-stationary eddies which entrain age, leading to relatively older water residing there.

Understanding Long‐Term Baseflow Water Quality Trends Using a Synoptic Survey of the Ground Water–Surface Water Interface, Central Wisconsin

The relationship between stream water quality and landscape activities is difficult to evaluate where the principal source of stream flow is ground water seepage because the average travel time from ground water recharge areas to stream discharge positions can be on the order of decades. We tested the idea that past and future baseflow water quality can be predicted based on a synoptic survey of ground water recharge age-dates (based on chlorofluorocarbon [CFC] measurements) and water quality measurements obtained at the ground water-surface water interface. In this study we (i) characterize the discharge-weighted age distribution and water quality of ground water seepage into the Little Plover River (LPR); (ii) use this information to backcast and forecast baseflow NO(3)(-) concentrations; and (iii) evaluate NO(3)(-) backcasts against historical baseflow data (1960 to 2000). The discharge-weighted apparent CFC age of ground water seepage into the LPR was 23.7 (+/-7) yr. Baseflow backcasts matched the four decade rise of baseflow NO(3)(-) from 2 to 8 mg L(-1). Baseflow forecasts included three scenarios. Scenario A projects the historical rise of NO(3)(-) in the LPR basin's ground water recharge through 2050. Scenario B projects a leveling off of NO(3)(-) in ground water recharge in the year 2000. Scenario C projects a leveling off in the year 1985. Under Scenario A, LPR baseflow NO(3)(-) will increase steadily from 8 to 19 mg L(-1) between 2000 and 2050. Under scenarios B and C baseflow NO(3)(-) will plateau at 13 mg L(-1) in 2030 and at 10 mg L(-1) in 2010, respectively. The approach developed in this study can be used to (i) reconstruct historical baseflow water quality patterns in the absence of long-term monitoring data and (ii) project the effects of potential management decision on future water quality.

Characterizing Transport between the Surface Mixed Layer and the Ocean Interior with a Forward and Adjoint Global Ocean Transport Model

Abstract The theory of first-passage time distribution functions and its extension to last-passage time distribution functions are applied to the problem of tracking the movement of water masses to and from the surface mixed layer in a global ocean general circulation model. The first-passage time distribution function is used to determine in a probabilistic sense when and where a fluid element will make its first contact with the surface as a function of its position in the ocean interior. The last-passage time distribution is used to determine when and where a fluid element made its last contact with the surface. A computationally efficient method is presented for recursively computing the first few moments of the first- and last-passage time distributions by directly inverting the forward and adjoint transport operator. This approach allows integrated transport information to be obtained directly from the differential form of the transport operator without the need to perform lengthy multitracer time integration of the transport equations. The method, which relies on the stationarity of the transport operator, is applied to the time-averaged transport operator obtained from a three-dimensional global ocean simulation performed with an OGCM. With this approach, the author (i) computes surface maps showing the fraction of the total ocean volume per unit area that ventilates at each point on the surface of the ocean, (ii) partitions interior water masses based on their formation region at the surface, and (iii) computes the three-dimensional spatial distribution of the mean and standard deviation of the age distribution of water.

Investigation of processes leading to nitrate enrichment in soils in the Kalahari Region, Botswana

In Southern Africa elevated nitrate concentrations are observed in mostly uninhabited semi-arid areas. In the Kalahari of Botswana groundwater locally exhibits concentrations up to 600 mg/l. It is assumed, that nitrate found in the groundwater originates mainly from nitrogen input and transformations in the soils. Our investigations in the Kalahari between Serowe and Orapa show that cattle raising is an important source for enhanced nitrate concentrations in the soils (Arenosols). But also in termite mounds very high nitrate stocks were found, and under natural vegetation (acacia trees and shrubs) nitrate concentrations were mostly unexpectedly high. This nitrate enrichment in the soils poses a serious threat to the groundwater quality. However, calculated soil water age distributions in the unsaturated zone clearly show that today’s nitrate pollution of the groundwater below the investigation area could originate from natural sources, but cannot be caused by the current land use for cattle raising.

Age and renewal time of water masses in a semi-enclosed basin - application to the Gulf of Finland

To estimate the water-renewal time of the Gulf of Finland, a numerical-model simulation comprising the years 1987 – 1992 was carried out for realistic meteorological forcing. The results concerning water age as well as renewal index proved to reflect properties of the mean circulation system. The highest water ages (of around 2 yr) were found in the south-eastern part of the Gulf. The water-age distribution after the 5-yr simulation proved to be almost identical to that obtained after 3 yr, indicating that the circulation in the Gulf may be rather stable. The analysis of the renewal-index results showed that it takes around 5 yr to renew 98% of the water masses of the Gulf. When local maxima of the renewal index are found in conjunction with high water ages, this may indicate that the region is particularly vulnerable to eutrophication.

Age and renewal time of water masses in a semi-enclosed basin — application to the Gulf of Finland

To estimate the water-renewal time of the Gulf of Finland, a numerical-model simulation comprising the years 1987 — 1992 was carried out for realistic meteorological forcing. The results concerning water age as well as renewal index proved to reflect properties of the mean circulation system. The highest water ages (of around 2 yr) were found in the south-eastern part of the Gulf. The water-age distribution after the 5-yr simulation proved to be almost identical to that obtained after 3 yr, indicating that the circulation in the Gulf may be rather stable. The analysis of the renewal-index results showed that it takes around 5 yr to renew 98% of the water masses of the Gulf. When local maxima of the renewal index are found in conjunction with high water ages, this may indicate that the region is particularly vulnerable to eutrophication.

Groundwater flow patterns in extensive shallow aquifers with gentle relief: Theory and application to the Galena/Locust Grove region of eastern Maryland

The Delmarva Peninsula between the Chesapeake Bay and the Atlantic Ocean comprises an irregular sequence of almost horizontal sand and silt or clay layers [Shedlock et al., 1999; Böhlke and Denver, 1995] whose connectivity and detailed geometry are not well known. A mosaic of groundwater basins with areas generally less than 30 km2 provide the water supply for the region. The concentration of dissolved nitrate has increased greatly since 1950 [Shedlock et al., 1999], and a series of test wells, drilled in the Locust Grove area for chemical analyses of the groundwater, provide useful data on water table levels at various locations. A new depth‐integrated hydraulic model of the aquifer flow is developed to handle the complex and poorly known structure of the region. It requires as inputs (1) historical rainfall data, (2) the elevation profiles along potential discharge streambeds, and (3) the depth‐integrated hydraulic conductivity, inferred from a few individual well water levels. The analysis predicts quantitatively and without arbitrary parameters, the distribution of groundwater heights above mean sea level and the patterns of total groundwater flow throughout the study region. Throughout the test area the predicted and measured groundwater levels generally agree to within 1 meter. The groundwater flow patterns show two distinctive types of domain with little intermingling, one type originating near the groundwater divide and discharging into the semipermanent streams and the other kind arising between the lower stream sections and discharging directly along the estuary shoreline. The measured distribution of water age with depth [Böhlke and Denver, 1995] agrees well with a simple formula provided by the theory.

Direct Simulation of Ground Water Age in the Rabis Creek Aquifer, Denmark

AbstractThe steady‐state ground water age distribution in the Rabis Creek Aquifer in Denmark is simulated directly and compared to the age estimated from observed tritium concentrations at three wells. The simulation code is based on an existing solute transport code, modified to include a zero‐order rate term of unity to account for the aging of ground water by one day per one day elapsed. The directly simulated ages of the observed tritium peaks agree to within less than four years with the observations. The tritium peaks would have been located to within 2 m vertically if the directly simulated ages had been used to select sampling depths in the three wells. An analytical solution that assumes that the geology is uniform and rectangular is shown to be a useful tool as a firsthand estimate of the age distribution. However, ages from a numerical direct age simulation that account for nonuniform aquifer properties agree better with the observed ages.

Ground Water Age and Nitrate Distribution Within a Glacial Aquifer Beneath a Thick Unsaturated Zone

AbstractThe impact on ground water quality from increasing fertilizer application rates over the past 40 years is evaluated within a glacial aquifer system beneath a thick unsaturated zone. Ground water ages within the aquifer could not be accurately determined from the measured distribution of 3H and as a result, chlorofluorocarbon (CFC) and 3H/3He dating techniques were applied. Beneath a 25 m thick unsaturated zone, ground water ages based on CFC‐11 concentrations were greater than 3H/3He ground water ages by 6 to 10 years, due to the time lag associated with the diffusion of CFCs through the unsaturated zone. Using the corrected CFC‐11 and 3H/3He ground water ages and the estimated travel time of 3H within the unsaturated zone, the approximate position of ground water recharged since the mid‐1960s was determined. Nitrate concentrations within post mid‐1960s recharge were generally elevated and near or above the drinking water limit of 10 mg‐N/L. In comparison, pre mid‐1960s recharge had nitrate concentrations <2.5 mg‐N/L. The elevated NO3− concentrations in post mid‐1960s recharge are attributed mainly to increasing fertilizer application rates between 1970 and the mid‐ to late 1980s. Anaerobic conditions suitable for denitrifkation are present within pre mid‐1960s recharge indicating that removal of DO is a slow process taking tens of years. Over the next 10 to 20 years, nitrate concentrations at municipal well fields that are currently capturing aerobic ground water recharged near the mid‐1960s are expected to increase because of the higher fertilizer application rates beginning in the 1970s and 1980s.

Patterns and Age Distribution of Ground‐Water Flow to Streams

AbstractSimulations of ground‐water flow in a generic aquifer system were made to characterize the topology of ground‐water flow in the stream subsystem and to evaluate its relation to deeper ground‐water flow. The flow models are patterned after hydraulic characteristics of aquifers of the Atlantic Coastal Plain and are based on numerical solutions to three‐dimensional, steady‐state, unconfined flow. The models were used to evaluate the effects of aquifer horizontal‐to‐vertical hydraulic conductivity ratios, aquifer thickness, and areal recharge rates on flow in the stream subsystem. A particle tracker was used to determine flow paths in a stream subsystem, to establish the relation between ground‐water seepage to points along a simulated stream and its source area of flow, and to determine ground‐water residence time in stream subsystems. In a geometrically simple aquifer system with accretion, the source area of flow to streams resembles an elongated ellipse that tapers in the down gradient direction. Increased recharge causes an expansion of the stream subsystem. The source area of flow to the stream expands predominantly toward the stream headwaters. Base flow gain is also increased along the reach of the stream. A thin aquifer restricts ground‐water flow and causes the source area of flow to expand near stream headwaters and also shifts the start‐of‐flow to the drainage basin divide. Increased aquifer anisotropy causes a lateral expansion of the source area of flow to streams.Ground‐water seepage to the stream channel originates both from near‐ and far‐recharge locations. The range in the lengths of flow paths that terminate at a point on a stream increase in the downstream direction. Consequently, the age distribution of ground water that seeps into the stream is skewed progressively older with distance downstream. Base flow ia an integration of ground water with varying age and potentially different water quality, depending on the source within the drainage basin. The quantitative results presented indicate that this integration can have a wide and complex residence time range and source distribution.

EXPLICIT CALCULATION OF WATER QUALITY PARAMETERS IN PIPE DISTRIBUTION SYSTEMS

Abstract Explicit algorithms have been developed for use in water quality studies and management of distribution networks with sources of dissimilar quality. The proposed algorithms can be effectively used for directly determining a variety of blended water quality parameters for a given set of network loading and operational conditions. The parameters include the spatial distribution of constituent concentrations, the flow influence characteristics of supply sources, and the water age distribution throughout the system. The developed algorithms are formulated analytically from mass balance relationships as contingent linear boundary value problems in conjunction with a topological sorting methodology. The resulting formulations yield coefficient matrices, of order equal to the number of junction nodes, that are triangular and positive definite. The parameter solutions sought are, thus, derived through a direct application of a one step substitution process. The proposed algorithms are both robust and eff...

Model calculations for transport of H-3 in groundwater systems

On the basis of studies of the behaviour in time of natural H-3 content in hydrological systems mathematical models can be developed for the water system being studied which permit extrapolation to the future. The purpose of this is to identify the transfer function and the relation-ship between input and output concentrations. Since the behaviour of water containing tritium and ordinary water is almost the same, this task is equivalent to determining the age distribution of water at the output of the hydrological system. The commonly used method is to calculate the output functions from the known input variations assuming different flow models and impulse responses. The model providing the best fit of measured and calculated output data is then adopted to describe the flow system. A number of models such as exponential and Gaussian distribution have been used to describe aquifer behaviour. Some examples of the application of model calculations of groundwater systems are discussed. For this, the tritium content of precipitation was used as the input function. The anticipated trend in concentrations at the output of the system was calculated for different mean residence times and compared to measured data.

The hydrology of the Jordan tributaries (Israel): Hydrographic and isotopic investigation

Nahal Dan, Nahal Snir and Nahal Hermon are the three principal tributaries of the Jordan River. All three are spring-fed and appear to derive their discharges from the same regional groundwater aquifer. The behaviour of this aquifer is examined by hydrograph separation and multivariate time-series analysis of discharge and rainfall records, and by investigation of 18O and tritium records. Hydrograph separation gives base-flow recession constants of 11–20 months and in addition N. Snir and N. Hermon show an interflow recession of ∼ 1 month. The dual reservoir system for these two tributaries is confirmed by time-series analysis. 18O data distinguish the different catchments for the springs and support the idea that interflow occurs in the near-surface part of the regional groundwater reservoir. Tritium data illustrate the inhomogeneity of the karstic reservoir. Assuming well-mixed conditions, the difference in behaviour between tritium records for rainfall and the tributaries is used to estimate the age distribution of water in the reservoir. At any time 50% of the storage is less than 1 yr. old and 90% is less than 3 yr. old.

Ground‐Water Age Distribution in Madrid Basin, Spain

ABSTRACTThe alluvial deposits that occupy the Madrid Basin in central Spain form an aquifer system covering an area of 5,000 km2 (2,000 mi2) and with thicknesses of 2,000 m (6,600 ft) or more. Average annual precipitation is 500 mm (15 in.) and average annual temperature is 15°C (59°F). The precipitation is sufficient to provide a net surplus to ground‐water recharge which, in turn, supports dry weather flow of major streams in the basin. A distribution of surface recharge and hydraulic conductivities were obtained from a previous study utilizing a two‐dimensional finite‐difference model of the same vertical cross section as this study. In this study a flow net and a discrete‐state compartment (or “mixing‐cell”) model were employed to calculate the age distribution of ground water circulating through the aquifer. Carbon‐14 decay ages were determined for nine ground‐water samples taken from eight locations. The ages obtained with the flow net and with the mixing‐cell models are mutually consistent and generally agree with the carbon‐14 decay ages. The calculated ages range from zero at the recharge boundaries to over 100,000 years at discharge boundaries in stream channels. The results obtained are to be regarded as preliminary. Their principal value will be to guide future C‐14 field sampling programs.